The expression "butyl rubber" as employed herein is defined to include a copolymer of a C.sub.4 to C.sub.6 isolefin, preferably isobutylene, and a C.sub.4 to C.sub.8 conjugated diolefin, preferably isoprene. A preferred butyl rubber contains from about 97 to about 99.5 weight percent of isobutylene and from about 0.5 to about 3 weight percent of isoprene.
The processes used to prepare butyl rubber are well known in the art. Commercially butyl rubber is prepared in a low temperature cationic polymerization process using Lewis acid type catalysts of which a typical example is aluminum trichloride. The process used most extensively employs methyl chloride as the diluent for the reaction mixture and the polymerization is conducted at temperatures of the order of less than -90.degree. C. Methyl chloride is employed for a number of reasons, one being that it is a solvent for the monomers and aluminum trichloride catalyst and a nonsolvent for the polymer product and thus affords a slurry. Also, methyl chloride has suitable freezing and boiling points that permit, respectively, low temperature polymerization and effective separation of the methyl chloride from the product polymer and unreacted monomers. However, it is also possible to conduct such polymerizations in a diluent which is a solvent for the polymer produced, examples of such diluents being the hydrocarbons pentane, hexane, heptane and mixtures of the aforementioned solvents with one another or with methyl chloride and/or methylene chloride. Recovery of the butyl rubber is achieved by conventional techniques used to recover rubbery polymers inducing (i) contacting the polymer slurry or solution with steam and water thereby flashing off the diluent (ii) passing the aqueous slurry through a vibrating screen or rotary filter to separate the bulk of the water and (iii) passing the polymer through a tunnel drier or a series of extruders or both to completely remove all the water and occluded gases.
Halogenated butyl rubber which can be either brominated butyl rubber or chlorinated butyl rubber is produced by the bromination or chlorination of the aforementioned butyl rubber. Brominated butyl rubber (bromobutyl rubber) typically contains from about 1 to about 3, preferably from about i to about 2, weight percent of isoprene and from about 97 to about 99, preferably from about 98 to about 99, weight percent of isobutylene, based on the hydrocarbon content of the rubber, and from about 1 to about 4, preferably from about 1.5 to about 3, weight percent of bromine, based on the bromobutyl rubber. Chlorobutyl rubber typically contains from about 1 to about 3, preferably from about 1 to about 2, weight percent of isoprene and from about 97 to about 99, preferably from about 98 to about 99, weight percent of isobutylene, based on the hydrocarbon content of the rubber and from about 0.5 to about 2.5, preferably from about 0.75 to about 1.75, weight percent of chlorine, based on the chlorobutyl rubber.
Commercially halogenation of the butyl rubber is carried out in a hydrocarbon solution such as hexane using elemental chlorine or bromine in a 1:1 molar ratio relative to the enchained isoprene in the butyl feedstock. The solution of butyl rubber with the desired molecular weight and mole percent unsaturation in hexane may be prepared by one of two procedures; one involving dissolution of the slurry from a butyl polymerization reactor and the other involving dissolution of solid pieces of finished butyl rubber. In the former procedure the cold slurry in methyl chloride is passed into a drum containing hot liquid hexane which rapidly dissolves the fine slurry particles. The methyl chloride and the unreacted monomers are flashed off for recovery and recycle and the hot solution is adjusted to the desired concentration for halogenation, typically from about 20 to about 25 weight percent butyl rubber in an adiabatic flash step. In the latter procedure bales of finished butyl rubber, chopped or ground to small pieces, are conveyed to a series of agitated dissolving vessels and solutions containing from about 15 to about 20 weight percent butyl rubber are obtained in from about 1 to about 4 hours depending upon the temperature, particle size and amount of agitation. In the halogenation process the solution of butyl rubber is treated with chlorine or bromine at a temperature of from about 40.degree. to about 65.degree. C. in one or more highly agitated reaction vessels, the chlorine being introduced as a gas or in dilute solution because of its rate of reaction with butyl rubber. Because of its lower rate of reaction bromine may be used in liquid or gaseous form. The hydrochloric or hydrobromic acid generated during the halogenation is neutralized with dilute aqueous base and the aqueous layer is subsequently removed by settling. Antioxidants or stabilizers are then added and the halogenated butyl rubber is then recovered in a manner similar to that used to recover butyl rubber.
Investigations of the molecular structure of the halogenated butyl rubbers have shown that in current commercial halogenation procedures a number of allylic halides are produced by means of an ionic mechanism wherein a positively charged halogen atom is added to the double bond of the enchained isoprene and a proton alpha to the carbonium ion is subsequently abstracted by a negatively charged species resulting in a shift in the double bond. The major halogenated isomer produced both in the case of chlorobutyl rubber and bromobutyl rubber has been found to be the exomethylene allylic halide isomer.
It is believed that these allylic halide structures in the halogenated butyl rubbers are the reason why the halogenated butyl rubbers exhibit enhanced cure compatibility with highly unsaturated elastomeric materials such as natural rubber, styrene-butadiene rubbers, polybutadiene rubbers and the like relative to butyl rubber.
In view of the foregoing it would be advantageous if halogenated polymers with allylic halide functionality analogous to the halogenated butyl rubbers could be synthesized directly by the copolymerization of the appropriate monomers thereby leading to a reduction in costs associated with the process of halogenating butyl rubber.